Thermionic ionization detectors are used in the field of chromatography for the detection of specific constituent components of a sample that is present in a fluid stream. Such detectors usually include a thermionic source having a surface impregnated with an alkali metal compound so as to make the detector specifically sensitive to a halogen, nitrogen, or phosphorus compound. An electrical heating current, carried by a resistive heating wire embedded in the thermionic source, heats the thermionic source. Certain sample compounds, or their decomposition products, extract the electrical charge from the hot thermionic surface of the source. Ions form on the surface of the thermionic source and migrate through a fluid stream flowing past the thermionic source to a collector electrode. The resulting ion current is collected at the collector electrode. An electronic current-measuring circuit, such as an electrometer, measures the ion current arriving at the collector electrode.
The ionization mechanism in these thermionic detectors is believed by some practitioners to be a surface ionization process rather than a gas phase process. (See, for example, Patterson, Journal of Chromatography, Vol. 167, p. 381, 1978.) Prior art thermionic detection techniques have therefore attempted to improve the construction and performance of the thermionic source. For example, U.S. Pat. No. 2,795,716 discloses a detector featuring a source in the form of a cylindrical alumina ceramic core upon which is wound a heater coil; U.S. Pat. No. 3,852,037 discloses the deposition of a material in the form of a bead onto an electrical heating wire to form a thermionic source.
Accordingly, FIGS. 1-A through 1-D represent the typical shapes and configurations of the ion collector (C), thermionic source (S), and sample inlet (I) in commercially available detectors. In FIGS. 1-A and 1-C, the source (S) is formed as an alkali-glass bead fused on a heating wire in the shape of a loop. In FIG. 1-B, the source (S) includes a heater wire wrapped on a ceramic core having an alkali-glass material fused over the outer surface to form a bead. In FIG. 1-D, the source (S) includes a sub-layer coating of ceramic cement and a non-corrosive, metallic compound additive, and a surface layer of a mixture of ceramic cement and an alkali metal compound additive, that are molded about a loop of heating wire to form a solid cylindrical bead. A conventional thermionic source is thus designed as a solid element that is positioned within the fluid stream. When the fluid stream is flowing, the majority of the contact of the fluid stream with the thermionic source occurs on the leading (upstream) portion of the exterior of the thermionic source.
The sensitivity of conventional thermionic ionization detectors is affected by changes in the interaction of the fluid stream with the thermionic source (S). For example, variations in the fluid stream flow rate, or erosion of the surface of the thermionic source, can alter the contact area of the fluid stream on the thermionic source (S). As a result, there is a variation in the amount of fluid that interacts with the alkali-metal compounds in the thermionic source and the accuracy and the sensitivity of the detector is not suitable for some applications.
Moreover, the alkali-metal compounds in the thermionic source are corrosive to the metallic heating wire that is typically employed to heat the alkali compounds; some samples include chemical components that are corrosive as well. Corrosion of the heating wire is known to cause detector failure and accordingly conventional approaches have attempted to decrease the exposure of the wire to corrosion. One such approach includes coating the metallic heating wire with a sub-layer comprised of non-corrosive ceramic material or a mixture of ceramic material and an inorganic, electrically conductive and non-corrosive chemical additive. See, for example, U.S. Pat. No. 4,524,047. However, the success of this approach depends upon the integrity of the coating; the development of voids or cracks in the coating during manufacture or operation can lead to corrosion.